Electronic synchronizing system for producing pitch discs and the like



June 24, 1958 E. M. JONES ELECTRONIC SYNCHRONIZING SYSTEM` FOR PRODUCINGFITCH DISCS AND THE LIKE Filed DBG. 30. 1949 5 Sheets-Sheet 1 o l." Ix/v e 0 u 0 MF. c

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E. M. JONES June 24, 1958 ELECTRONIC SYNCHRONIZING SYSTEM FOR PRODUCINGPITCH DISCS AND THE LIKE Filed D60. 30, 1949 3 Sheets--Sheet 2 INVENTOR.

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June 24, 1958 E. M. JONES ELECTRONIC SYNCHRONIZING SYSTEM FOR PRODUCINGFITCH DISCS AND THE LIKE Filed Dec. 50, 1949 P SYNCHRaA/aus PULSE: Feen'use GENE/Mraz 5g, er a. Z

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/n WMM/lug ATTORN EYS niteti States Patent() ELECTRONIC SYNCHRNIZINGSYSTEM FOR PRODUCNG PTCH DISCS AND THE LIKE Edward M. Jones, Cincinnati,Ohio, assignor to The Baldwin Piano Company, a corporation of Ohio Myinvention relates generally to the synchronization of a relatively lowfrequency electronic oscillator with a source of relatively highfrequency oscillations so that the frequency of the former is asubmultiple of the frequency of the latter. In particular, my inventionrelates to methods and means for causing one revolution or cycle ofmotion of a moving object, such as a turntable to occur during anintegral number of cyclic variations in intensity of a light source.

The particular uses to which I have put my invention are: (1) producingpitch-determining scanning discs for photoelectric musical instruments,(2) producing voice (wave form) discs for such instruments, and (3)dividing a circle into any of a wide range of equal parts. It will beobvious to those skilled in the art that my invention has many otheruses, such as producing scales for protractors, azimuth indicators anddevices requiring the division of a given distance into equal parts orrecurring patterns. Also, my invention may be used in electricalcircuits in which a plurality of frequencies must be maintained inratios of large prime numbers.

Although l shall describe my invention with respect to the threespecific uses mentioned above, it will be understood that the appendedclaims shall be construed broadly to cover methods and means forproducing equally divided re-entrant paths containing integral numbersof cyclic patterns and for graduating circles, scales and the like.

Certain aspects of my invention are specifically claimed in applicationSerial No. 436,831, led .Tune 15, 1954, entitled ElectronicSynchronizing System for Producing Pitched Discs and the Like, which isa continuation-inpart of this application.

It is well known that musical tones can be produced by means of a systemwherein, iirst, a beam of parallel light is directed through a series ofplaying-key-openable apertures selectively exposing small,circumferential segments of concentric rows of transparent areas in anopaque pitch disc rotated at a speed such that areas in respective rowspass their respective apertures at rates corresponding to thefundamental frequencies of the tones desired; second, the moving rays oflight thus produced are caused to scan wave form patterns of a variableopacity or variable area type; and third, the varying rays are directedupon a photocell in an appropriate circuit including electroacoustictranslating means. It is also known that to produce higher orderharmonics of a given complex tone, the scanning rays must be extremelynarrow, thus requiring that transparent areas in the pitch disc benarrow radial slots.

Extreme accuracy is required in locating the transparent slotsequidistantly around the respective rows of a pitch disc. Randomvariations in the distance between adjacent slots are productive ofextraneous frequencies resulting in low signal-to-noise ratios, whilecumulative errors in spacing around a row of slots result inlowfrequency modulation, commonly known as wow.

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Also, those skilled in the art know that through a given aperture aplurality of cycles of wave-form patterns may be scanned simultaneouslyby an equal number of transparent slots and that the respective cyclesof wave-form patterns must be spaced exactly the same as the transparent.slots which cause light beams to scan them.

As disclosed by Armand F. Knoblaugh in a copending application entitledMusical instruments Employing Continuously Moving Members, Serial No.39,674, filed July 20, 1948, now U. S. Patent No. 2,586,664, grantedFebruary 19, 1952, practical pitch discs for photoelectric musicalinstruments may be of the order of one foot in diameter. Discs of thistype require many rows of slots, some of which have prime numbers ofslots of the order of 1000 and less for approximating the equallytempered scale. In making such a disc, a first criterion is an equalspacing of a given number of slots about the specific circumference of agiven pitch track. The slots must be so spaced as to come out even,since otherwise there will be extraneous noise. A second criterion isaccuracy in the spacing of the slots. My experience with such discsindicates that random variations in the distance between adjacent slotsof the order of .0001 inch, obtainable by another method I haveemployed, produces a signal-to-noise ratio which is lower than desirablein high-quality electronic musical instruments.

Previously employed methods of producing pitch discs have either lackedthe accuracy desired or have required large amounts of time for theirproduction. In the practice of one phase of my invention I rotate atransparent disc or other element having a photosensitive coatingthereon by means of a synchronous motor energized from a source thefrequency of which can be altered from a nominal one as required to makeone revolution correspond exactly in time to that of an integral numberof pulses of energy supplied to a light source which is used to producethe images of the slots upon the photosensi.- tive coating for exposurethereof.

My invention, in brief, comprises a system by means of which a widerange of dividing ratios between the frequency of the light pulses, orvariations and the speed of the motor, is available so that the desirednumber of photographically produced slots or cycles of wave forms may beproduced in the respective concentric rows. During the exposure of agiven row, a disc may be rotated a number of times in order that theindividual cyclic areas may receive suticient light to expose thephotographic emulsion. It can readily be seen that these requirementsnecessitate an extremely stable and precise system. The speed of themotor must be extremely constant and the dividing ratio between thefrequency of light variations and the frequency of the energy suppliedto the synchronous motor must be an exact integer.

A broad object of my invention is to provide methods and means fordividing re-entrant paths into integral numbers of cyclic patterns.

A still further object of my invention is to produce a voice disc for aphotoelectric musical instrument.

An object of my invention is to provide means for synchronizing thefrequency of the output of a variablefrequency harmonic synthesizer andthe speed of a synchronous motor so as to produce an integral number ofcycles of a desired wave form during each revolution of a member drivenby the motor.

Another object is to provide a harmonic synthesizer which will supply acomplex wave, the harmonics of which are variable as to amplitude andphase with respect to the fundamental and the frequency of which can bevaried as desired without appreciable change in harmonic content.

These and other objects of the invention, which will be set forthhereinafter or will be apparent to one skilled in the art upon readingthese specifications, I accomplish by those certain constructions,circuits and arrangements of parts of which I shall now describeexemplary embodiments.

Reference is made to the accompanying drawings wherein:

Figure 1 shows a block diagram of a basic system .for obtaining onerevolution of a turntable during an in-Y tegral number of pulses oflight.

Figure 2 is a block diagram of a portion of the system of Figure 1, suchas I employ for synchronizing the speed of a turntable with cyclicvariations of light other than pulses.

Figure 3 is a block diagram of the same portion of the system of Figure1, such as I employ for synchronizing the speed of Va turntable withpulses of light.

Figure 4 is a block diagram of the essential elements in a synchronizingsystem according to my invention.

Figure 5 is a graphical representation of the wave form of voltageacross'the tuned circuit of the low-frequency oscillator of Figure 4.

Figure 6 illustrates the wave form of the output of the direct-coupledamplifier of Figure 4.

Figure 7 is a diagram, partly schematic, partly block, of a completesystem for graduating a circular path.

Figure 8 shows a graph of the relationship between the reading of a peakvoltmeter and the frequency of a high frequency source.

Figure 9 illustrates an aperture system such as may be employed with myinvention.

Figure 10 shows a portion of a pitch disc having a coradial series ofslots.

Figure ll represents part of a series of graduations such as may beproduced by the system of Figure 7.

Figure 12 represents a series of indexed graduations.

Figure 13 shows exaggerated results of a method by which the graduationsof Figure 12 may be produced.

Figure 14 shows a plurality of wave form patterns such as may beproduced by the system of Figure 15.

Figure '15 is aediagrammatic representation of a harmonic synthesizerand a synchronizing circuit therefor.

Figure 16 illustrates a synchronous motor and turntable assembly.

The basic problem The basic problem to which the hereindescribedexemplary embodiments afford a practical solution, is to cause onerevolution or cycle of motion of an element to occur in exactlythe sameamount of time required for a preselected number of electric pulses orof cycles of a cyclic electric wave of desired form generated by aseparate source. Ancillary to the basic problem is the problem ofcausing a light source to vary in intensity in accordance with thepulses or with the variations in amplitude of theelectric wave.

General method The solution to the basic problem lies, according to myinvention, in the following general method. First, I produce relativelylow-frequency oscillations at a rate which determines the rotationalspeed of a synchronous motor driving the moving element. Second, Igenerate relatively high-frequency oscillations which can be varied overa range of frequencies depending upon certain requirements to bediscussed in detail hereinafter. Third, by a series of dividers, Idivide the high frequency oscillations by consecutive integral powers ofthe dividing ratio of the dividers. Fourth, I select from the series ofdividers a particular divider having a frequency range over the range ofadjustment of the high frequency generator) in which there occurs thatparticular frequency which vwill produce the desired number of cyclesduring one revolution of the element driven by the synchronous motor.Fifth, I synchronize the frequency of the lower 4 frequency oscillationswith that of the higher frequency oscillations so that the former isexactly a submultiple of the latter. Sixth, I modulate the intensity ofa light source at the rate selected in step four of this method.

If the modulation of the light source is to be in the natureof a pulse,I generate a pulse of a desired shape at the frequency selected in stepfour above.

If the modulation of the light source is to be in the nature of a sinewave, or of complex variations, I first generate oscillations of thedesired wave form and then synchronize their fundamental frequency withthe frequency of the pulses generated at a rate selected in step fourabove.

The basic system Briey, in the practice of my invention, I employ arelatively yhigh-frequency, variable-frequency oscillator 1 (Figure l)which is continuously variable over a range of at least two-to-one. Asecond oscillator 2 is tixedtuned at a relatively low-frequency whichcan be altered slightly by the injection of a synchronizing pulse fromthe synchronizing circuit 3 into its tuned circuit during each cycle ofoscillation. By proper injection of this pulse, it is possible to makethe frequency of the L. F. oscillator 2 a submultiple of the frequencyof the H. F. oscillator 1, no matter what the frequency of the lattermay be within its range.

My invention in one of its broader aspects concerns itself with what Ibelieve is a novel system for introducing the synchronizing pulses justreferred to. However, in the specific uses which I make of the system Imay employ additional circuits in connection with the oscillatorsdescribed above.

First, I may use the H. F. oscillator 1 to trigger a series of frequencydividers collectively designated at 4, each dividing by two thefrequency of the preceding one. Second, a pulse generator 5, connectableby a selector switch 6 to one of the dividers 4 for a desired dividingratio, is triggered by that divider and determines the frequency atwhich a light modulator 7 modulates the intensity of a light source 8,such as a crater lamp. Third, the L. F. oscillator 2 may be used todetermine the frequency of a wide-pulse .generator 9 which supplies,

through a power amplifier 10, pulsating direct current to a synchronousdriving means, such as a motor 11 driv-V ing a turntable 12, upon whichmay be mounted a photosensitive plate to be converted to a pitch disc orvoice disc, or upon which a divided circle is to be formed.

The general system of Figure l, just described, provides means wherebyone revolution of a turntable will occur during a desired whole numberof cyclic variations in the intensity of the light source.

When I desire to produce during each revolution of the turntable 12 anintegral number of cycles of a specific wave form, the light modulator 7of Figure l comprises those elements of Figure 2 included in the areabound by the dashed line 7a-namely, a variable-frequency, variablewave-form, harmonic synthesizer 13, the fundamental frequency of thewave desired therefrom being synchronized by means of a synchronizingcircuit, indicated generally at 14, with the pulses provided by thepulse generator 5 via the point 15a, which in this case is connected tothe pulse generator 5 at the point 1S. The synthesizer may includeamplification means for supplying to the lamp 8 sulicient energy toproduce a desired level of intensity.

If I desire to produce during each revolution of the turntable 12 anintegral number of pulses of light I simply employ in the place of thelight modulator 7 a power amplifier 16 (see Figure 3), the terminal 15bbeing connected to the pulse generator 5 at the point 15.

The oscillator synchronizing system The synchronizing circuit -3ofFigure preferably comprises the general circuit divisions shown inFigure 4 in the area surrounded by the dot-dash line 3a, although in thebroader aspects of my invention I am not limited thereto. It will beobvious as these specifications are read that other synchronizingcircuits could be employed.

Referring to Figure 4, the relatively high frequency oscillator 1a maybe continuously variable, if so desired, over a range of frequenciesdetermined by the dividing ratio of dividers such as 4 (Figure l), aswill be explained in detail hereinafter. The relatively low frequencyoscillator 2a, preferably of the L-C type, is fixed-tuned at a frequencyfn suitable for a desired purpose, such as determining the speed of asynchronous motor. The frequency of this oscillator 2a can be alteredslightly by the injection of a synchronizing pulse into its tunedcircuit during every cycle of its sine-wave oscillation. When this isdone, the wave form 20 of the voltage across this tuned circuit willhave a notch in it as indicated at 21 in Figure 5, effectively alteringit to a different frequency. By the proper introduction of thesynchronizing pulse, it is possible to make the L. F. oscillator 2aoperate at a frequency which is a submultiple of the frequency of the H.F. oscillator 1a. Since the frequency imposed on the light source 8 fromthe selected divider is also a submultiple of the frequency of the H F.oscillator, it will be evident that the frequency of the light sourceand the altered frequency of the L. F. oscillator 2a will be related asa whole-number ratio.

The method of injecting the synchronizing pulse will be explained ingeneral first, the circuit details being described` hereinafter. Thesine-wave voltage from the L. F. oscillator is fed through adirect-coupled amplier 22 (Figure 4), the output of which would normallybe substantially a square wave, due to overloading, as indicated by thedashed curve 23 in Figure 6. The actual output of the amplifier 22,as-represented by the solid line 24, is a pulse, because of the feedbackof a blocking oscillator 25 (Figure 4) discussed in detail hereinafter.

Trigger pulses from the H. F. oscillator 1a are fed to the blockingoscillator 25 along with the output of the direct-coupled amplifier 22via an amplifier-and-mixer 26. The bias on the blocking oscillator 25 isadjusted so that it will not be triggered by a pulse from the H. F.oscillator la until the pulse 24 from the direct-coupled amplifier 22has started. Since the beginning of the pulse 24 occurs at a certaininstant in each cycle of the L. F. oscillator 2a., the blockingoscillator 25 will fire on the first trigger pulse from the H. F.oscillator 1a after the instantaneous voltage of the L. F. oscillator 2apasses through a certain value (in a positive direction).

Thus the period of the blocking oscillator 25 is always about l/fn andan exact multiple of the period of the H. F. oscillator (where fn, asmentioned above, is the resonant frequency of the tuned circuit of theL. F. oscillator 2a). However, the former period would alternate betweentwo adjacent values if means were not provided to regulate itautomatically.

The period of the L. F. oscillator 2a is altered during each cycle bycombining the output of the direct-coupled amplifier 22 with a pulsefrom the blocking oscillator 25 and feeding it through a diode 27 to thetank circuit of the L. F. oscillator 2a. The blocking oscillator pulseputs a small negative charge into the condenser in the tuned circuit ofthe L. F. oscillator 2a forming the notch 21 in the curve 20 anddelaying the progress of the wave slightly, so that its frequency isequal to that indicated by the dashed sine wave 20a, representing afrequency which is a submultiple of the frequency of the H. F.oscillator 1a. The size of the notch 21 depends upon the instantaneousvalue of the voltage from the direct-coupled amplifier 22. If, for adesired ratio between the frequencies of the high and low frequencyoscillators, the trigger from the H. F. oscillator is slow in coming,the pulse from the direct-coupled amplifier 22 is given a chance tobuild up to a more negative value so that when the blocking oscillatorpulse does occur, a larger ratio of frequencies is obtained. As thefrequency of the H. F.y oscillator 1a is lowered, there occur suddenrises in frequency of the L. F. oscillator 2a as the blocking oscillator25 suddenly shifts to firing on an earlier trigger, and the size of thenotch 2l in the curve 20 suddenly shifts from a maximum to a minimumvalue.`

If the frequency of the H. F. oscillator 1a is varied, transitions fromone ratio to another occur as the frequency of the H. F. oscillator istuned throughout its range.

Circuit details of the oscillator synchronizing system The circuitdetails of the oscillator synchronizing means are presented in Figure 7,which shows a complete system for graduating a circular path. (Portionsof the system which correspond to those previously described, have thesame number but a different suffix letter; for example, the block 3 inFigure l represents the synchronizing system, whereas in Figure 4, thesynchronizing system comprises elements enclosed by the dot-dash line3a.) l shall first describe the circuit, thereafter explaining itsoperation in detail with reference to making a pitch disc or dividing acircle.

ln Figure 7 a variable H. F. oscillator 1b, preferably of theelectron-coupled type, known in the art, is provided with a tuning dial30 for changing the frequency thereof. The pulsed output of theoscillator 1b is connected as shown to a series of frequency dividers4a, 4b, 4c, etc., preferably multivibrators of the hip-flop type, eachdividing by two the frequency of the pulses fed to it. Either theoscillator 1b or a divider such as 4a may be used as a source ofvariable frequency oscillations and may be connected as at 31 and 33 toa selector switch 32, for selecting a desired range of dividing ratios.

The dividers 4b, 4c, and 4a are respectively connected to terminals 34,35 and 36 of a selector switch indicated generally at 6a. The commonterminal 37 of the switch 6a is connected to a pulse generator 5a, whichin turn is connected to a power amplifier 16a. The output of the poweramplifier is connected to the electrodes of a light source 8c, such as acrater lamp. A light 'beam from the crater lamp is directed toward anaperture system comprising an opaque member 38 having a transparent slot39, through which a ray passes and isconverged by a condensing lens 40toward a second lens 41. A minute image of the slot 39 is produced bythe second lens 14 upon the surface of the disc 42, which may be coatedwith a photosensitive emulsion and may be located on a turntable 12adriven by a synchronous motor 50, described below.

I may, for reasons hereinafter given, employ an additional frequencydivider 43 connected as shown by means of a selector switch 44 to theother dividers such as 4b, 4c and 4d. The divider 43 may be connectedthrough a switch 45 to another pulse generator 46, the output of whichmay be fed to an amplifier 47, thence to a second crater lamp 48. Bymeans of a mirror 49 the beam from the lamp 48 may be diverted anddirected through the aperture 39 and lens system to the plate 42.

The `contact arm of the switch 32 is connected as shown to an amplifierand mixer circuit 26a, the output of which is connected to the grid of avacuum triode T1, which acts as a cathode follower. Tubes T1 and T2,together withtheir associated components, comprise the blockingoscillator group 25a. The cathode of tube T1 is grounded through theresistor R1 and is connected through the winding L3 of a transformerindicated generally at 51 to the grid of the blocking oscillator tubeT2. The plate of tube T2 is connected through the winding L1 of thetransformer 51 to a positive plate potential V3 at the point 52. Thecathode of tube T2 is connected through a resistor R2, paralleled by acapacitor C1, to a positive bias potential V1, lower than the potentialV3. The winding L2 of the transformer may be connected as shown to atrigger amplifier 54, the output of which is connected to a wide-pulsegeneratorSS, which in turn is connected to the power amplifier S6. Thewinding of the synchronous motor 50 is energized from the poweramplifier 56. Although the preferred type of motor employed by me in thepractice of my invention will be discussed in detail hereinafter, itshould be stated at this point that I use a direct drive as one ofseveral features to obtain as nearly constant speed as possible.Consequently, by way of example, if I desire to run the turntable 12a ata speed of the order of 2 R. P. S. by means of wide pulses at afrequency of the y/order of 256 C. P. S., I will need 128 teeth on therotor and a matching set of 128 teeth on the stator.

The cathode of the tube T2 is also connected to the cathode of a vacuumtriode T3 which, together with Y indicated generally at 2b, andcomprising vacuum triodes Y T5 and T6 and associated components. Theplate of the tube T3 is connected to the grid of a cathode followertriode T4. Positive plate potential V3 for the tubes T3 and T4 may beconnected at the point 58, the resistor R4 being in series with theplate of the tube T3. The cathode of the tube T4 is connected to groundthrough a resistor R5 paralleled by capacitor C2. The output of thecathode follower is carried to a common point 59, from which aconnection is made to the amplifier and mixer circuit 26a through aresistor R6 and capacitor C3. The potential at the point 59 is measuredby a peak voltmeter 60. Point 59 is connected also through a capacitorC8 to the cathode of a vacuum diode T7, which is biased through aresistor R14 at a positive potential V2 of an amount between V1 and V3above. The plate of T7 is connected through the lower section 61 of adouble-pole, double throw switch 62 and through the winding L4 of thetransformer 51 to a resonant circuit comprising a capacitor C4 and aninductive element L5 in the L. F. oscillator 2b. The other section 63 ofthe switch 62 provides means for connecting the resonant circuit L5-C4to ground through a variable capacitor C7. The other side of the circuitL5-C4 is connected along with the cathode of the oscillator tube T5 tothe potential V1 at the point 65. The coil L6, which is inductivelycoupled to L5, is connected in series with the resistor R13 between theplate of tube T5 and the positive potential V3.

The grid of tube T5 is connected as shown in Figure 7 through a networkconsisting of capacitor C6, and resistors R9, R10 and R11 to the cathodeof tube T6, acting as a cathode follower, the grid of which is connectedto the tuned circuit LS-C4. The grid of the direct-coupled amplifiertube T3 is connected to the grid of tube T5 through resistor R8 andcapacitor C5 as shown. The grid of tube T3 is connected also to groundthrough resistor R3 and to the plate of tube T5 through resistor R7.

The cathode of the tube T6 is connected also through aphase-shiftnetwork 70 to one of the horizontal and to one of the vertical deectionplates of a cathoderay tube T8. Also connected to the same verticaldeection plate are capacitor C10, which transmits a pulse from the pulsegenerator 5a, and capacitor C11, which transmits a pulse from theblocking oscillator 25a. The other horizontal and vertical plates, alongwith the second anode of the tube T8, are connected to the relatively 8high potential V4. A focus potential is supplied inthe usual manner at71. A potential between the grid ,and cathode of the tube T8 for varyingthe intensity of the trace is provided by the potentiometer R12 inseries with the source of potential V1. A series resistor R18 isconnected between the variable contact of the potentiometer and the gridof the tube T8, this grid being connected also to the plate of a vacuumtriode T9. The grid of tube T9 is connected through a capacitor C9 toVthe fixed set of plates of a variable capacitor C12, the movable platesof which are mounted upon the turntable 12a, which is electricallygrounded. Connection to the positive potential V4 for the capacitor C12is made through a resistor R17, as shown. The cathode and grid of tubeT9 are respectively connected through resistors R15 and R16 to ground asillustrated in Figure 7.

An exemplary set'of values for the elements in Figure 7 is listed below:

R1 ohms 27,000 R2 do 18,000 R3 megohm 1 R4 do 1 R5 ohms 30,000 R6 do100,000 R7 megohms 1.8 R8 ohms 82,000 R9 do 3,400 R10 do 27,000 R11 do27,000 R12 do 650,000 R13 do 66,000 R14 do 47,000 R15 do 10,000 R16megohms 2.2 R17 do 4.7 R18 ohms 130,000 C1 rnf .01

C2 mmf 800 C3 rnmf 300 C4 mmf-- 12,500 C5 mmf 1,500 C6 mmf 2,200 C7 mmf140 (max.) C8 mf .01 C9 mf .0034 C10 mmf 25 C11 mmf 25 C12 mmf 15 (max.)L1 turns-- 167 L2 do 42 L3 do 83 L4 do 84 L6 henn'es 8 L6 do .2

T1 6SN7 (1/2) T2 6SN7 (1/2) T3 6SL7 (1/2) T4 6SN7 (1/2) T5 6SN7 (1/2) T66SN7 (l/2) T7 6H6 (1/2) T8 902A V1 volts 95 V2 do 165 V3 do 275 V4 do420 Crater lamp-R1130B (Sylvania) No. of teeth in synchronous motor-1284 Nominal frequency of L. F. oscillator 2li-500 C. P. S.

Frequency range of H. F. oscillator 1b-256 to 512 kilocycles sec.

Nominal frequency of pulses fed to motor-250 C. P. S. (employing as thepulse generator 55 a multivibrator circuit, dividing by two its inputfrequency) '9 Width of pulses fed to motor- 1700 microseconds Nominalspeed of synchronous motor 50-2 R. P. S.

(approx.) (more accurate]v ?.50/ 128) I shall next explain the operationof the system shown in Figure 7. The sinusoidal potential developedacross the tuned circuitl L-C4 is applied to the grid of the cathodefollower tube T6 resulting in a sinusoidal potential at the cathode oftube T6. The potential on the grid of the tube T5 would be a sine waveexcept that the positive half is flattened due both to grid current flowand to the fact that R16 is large enough to prevent excessive gridcurrent. Feedback potential for sustaining the oscillations in the tunedcircuit L5-C4 is obtained from the plate circuit of tube T5 by inductivecoupling between the windings L6 and L5.

A signal is fed from the L. F. oscillator through the resistor R8 andcapacitor CS to the grid of the tube T3 in the direct-coupled amplifier22a, the purpose of which is to provide a steep-front wave occurring atthe instant that the sine-wave oscillation in tuned circuit L5--C4passes through the zero point in a cycle. Resistors R9 and R11 act as avoltage divider for the purpose of fixing the D. C. bias Aon the grid oftube T3 at a point which permits tube T3 to start conducting at theexact half-cycle point in a given sine-wave of oscillation of thecircuit L5-C4. This prevents the L. F. oscillator from oscillating atdifferent amplitudes due to changes in amplitude of the synchronizingpulses which will be discussed below. After the plate potential of thetube T3 has gone to quite a low value it is desirable to terminate thepulse and somehow bring the wave back positive again. This isaccomplished by two means. First, since the cathode of the amplifiertube T3 is connected directly to the cathode of the blocking oscillatortube T2, the instant that oscillation starts therein, a positive voltageoccurs on the cathode of tube T2 and also on the cathode of tube T3,thus cutting the tube T3 off. However, this positive voltage on thecathode of the T 2 tube does not last very long and additional means forkeeping the direct-coupled amplifier tube T3 cut 'off s employed. Thissecond means is the resistor R7 which feeds to the grid of the tube T3from the plate of the oscillator tube T5 a potential which accomplishesthe desired result of keeping Vthe tube T3 cut off after the blockingoscillator pulse has subsided. The output of thev tube T3 is fed to thecathode follower tube T4. The

wave form hence is a negative pulse, terminated on its decay side asexplained above. The output of the cathode follower tube T4 is fedthrough resistor R6 and capacitor C3 to one of the input points of theamplifier and mixer 26a,-the output of which triggers off the blockingoscillator tube T2 through the cathode follower tube T1. However, thetrigger is not effective until the amplifier and mixer 26a also receivesa trigger from the oscillator 1b, or from a divider such as 4a, via theselector switch 32. Hence, the blocking oscillator tube T2 will fireafter the pulse on the cathodefollower tube T4 has started occurring,but not until a trigger pulse is received via the selector switch 32.

When theblocking oscillator fires, a voltage is induced across thewinding L4 and conduction occurs in the diode T7, causing the notchmentioned above to occur in the sine wave across the oscillator windingL5.

The size of the notch depends upon what the magnitude`v of the negativepulse on the cathode of the tube T4 is at the instant the blockingoscillator-fires, If the 'trigger from the high frequency oscillator islate, the notch is greater due to the fact that the end of the windingL4 marked is at a more negative potential. Because the notch is greater,the progress of the sine wave is delayed more so that the next triggerwill not occur any later with respect to the sine wave.

A quantity depending upon the size of the notch in the sine wave ismeasured by the peak voltmeter 60.

.1() The peak voltmeter measures the negative peak of the voltage onthe' cathode follower tube T4. The reason why the negative peak issignificant is that it shows how late the trigger is working, because assoon as the trigger occurs, the build-up of the negative pulse on thecathode follower is terminated.

The sine-wave oscillation occurring across the coil L5 is caused toproduce a circular pattern on the scope T8 by means of a phase-shiftnetwork 70 which feeds voltages degrees out of phase to the deflectionplates. A series of vertical markers, such as pips, is superimposed uponthe circular pattern by pulses from the pulse generator 5a through acapacitor C10, the switch 37 being setto a standard position and thedial 30 being adjusted so that a recognizable scope pattern is obtained.The recognizable scope patterns will occur when ever the frequency ofthe pulses produced by pulse generator 5a is an integral-multiple of thefrequency of the L. F. oscillator 2b.- When the desired dividing ratiodiffers from one of these ratios, the H. F. oscillator is first adjustedto give the nearest dividing ratio which has a recongnizable scopepattern; then the dial 30 of the H. F. oscillator is slowly turned inthe proper direction until the peak voltmeter 60 has indicated a numberof transitions equal the difference between the desired ratio and theabove-mentioned nearest ratio producing the recognizable Scope pattern.For example, if the ratio between the frequency of the H. F. oscillator1b and the frequency of the L. F. oscillator 2b is to be 899 and thenearest ratio producing a recognizable pattern is 896, then the dial 30is adjusted until the recognizable pattern is obtained (it mightcomprise, for example 14 pips superimposed up the circular pattern). Thedial is then turned in the increasingfrequency direction until threetransitions are made as observed on the peak-voltmeter 66. Referring toFigure 8, the saw-tooth characteristic shows an exemplary relationshipbetween the reading of the peak voltmeter 60 and the setting of the dial30. If the dial is set betweenfl and f2 the dividing ratio will be 896.As the dial is turned in the higher frequency direction, threetransitions from the maximum to the minimum peak voltmeter reading willoccur before the desired ratio of 899 is obtained. This ratio will bemaintained so long as the dial is set between f3 and f5. When it isdesired to keep such aratio fora considerable time, the dial should beset at f4, halfway between f3 and f5, which corresponds to the peakvoltmeter reading e2, halfway between the extremes el and e3, so thatany drift due to variations in temperature, line voltage, etc. will notallow the system to shift to an adjacent dividing ratio.

j It will be clear that one highly advantageous feature of my inventionis that minor shifts in the frequency of the H. F. source will notaffect the dividing ratio. For example, if the H. F. oscillatorfrequency shifts to a slightly lower value, such as between f3 and f4,the peak voltmeter reading will increase, indicating, as explainedabove, that a larger delaying charge is being introduced into the L. F.oscillator, reducing its frequency until it is exactly 1/899 of thefrequency of the H. F. oscillator. Also, without losing synchronism, itis possible to readjust'the high frequency oscillator to bring the peakvoltmeter reading back to the mean value e2 before a shift to anadjacentdividing ratio occurs.

I shall now describe the manner in which the system of Figure 7 can beemployed to produce two or more concentric series of areas on a disc 42,an area in one series being' coradialwith one area in the other series.First, l synchronize the system so that I may expose a desired number ofareas in the first of the two series. Next, I throw the switch 62 to theupper position, so that no synchronizing pulses are introduced into thetuned circuit L5-C4 of the L. F. oscillator 2b. Then I adjust thecondenser C7 until the frequency of the L. F. oscillator'7b is almost insynchronism with the H. F. oscillator 1b. This is evidenced by the slowmovement of the pat- 1 tern on the scope T8V produced by the pulses frompulse generator 5d. During each revolution of the turntable 12a.`theplates of -the capacitor C12 will mesh and cause a cut-oit of the platecurrent in the tube T9, which has been conducting (due to the potentialdropin the 'righthand portion of potentiometer R12), thuscausing thegrid of tube T8 to be at a potential lower than that at the movableterminal of potentiometer R12. Thus the cut-Oifof plate current iu tubeT9 will result in an increase i'n the intensity of the scope T8 once ineach revolution of the turntable 12a, during the time that the condenserplates are enmeshed. The plates must be o'f such size that the angulardisplacement of the disc which occurs during the meshing of the platesis less than the disc displacement from one set of motor teeth to thenext. Thus, since the rotor o'f the synchronous motor is allowed todrift (-by not being in synchronism), eventually one of the-many movingpips caused by pulses from the generator 5a will be superimposed uponthe one xed pip (the circular trace and this pip are derived from theVsame L. F. source) on the scope caused by the pulse from the winding L2of the blocking oscillator 25a during the intensified period when theplates of capacitorY C12 are enmeshed. At this instant the switch 62 isthrown to the synchronizing (lower) position and pulses from thegenerator 5a Will remain superimposed upon the pulse from the blockingoscillator occurring on the intensified trace. After an exposure hasbeen made for producing 'the iirst series of areas, the aperture 39 orthe whole optical system is shifted to the position corresponding to theradius desired for the second series of areas.

A structure, such as that illustrated in Figure 9, may be usedconveniently for shifting the aperture. The slider 3821 has an apertureV39a of desired dimensions, and is movable with respect to the member3817' which has a longer aperture 39h.

If a different number of areas is desired in the second Y series, thenew dividing ratio is selected and the system is made to occur at thisinstant, it will produce an vil'nag'e'on a particular radial line whichwill always be Vthe'saine for the various rows of areas exposed. Thus itis possible to establish coradiality between two areas or graduations indiierent concentric series. An example of coradial relationship isillustrated in Figure l0, wherein a portion 66 of an opaque pitch discfor a photoelectric musical instrument has coradial transparent slots67, 67a and 67b in three adjacent concentric rows. `1 Y The abovedescription referring to Figures 7 and'9 has been concerned with theproduction of one or more circular series of equal length graduations ona disc.V In Figure ll is shown a series of such graduations 72; I`norder to meet certain of the objects of my invention, 'I provide meansfor causing'certain regularly-spaced"grad-v uations to be longer thanthe others for indexing purposes. An example of such a series isillustrated in Figure 12. Graduations such as 73 are longer thanrthosedesignated as, but not as long as those numbered 74. I can indexgraduations by either of the following systems.

First, I may produce the extended portions of the longer graduations byexposing these areas simultaneously with the areas of which they are anextension. InvFigure 13 is illustrated a series of graduations producedby this method. However, the spaces between the extended porg: tions 73aand the main portions 72a has been exaggerated for the purpose ofillustrating the technique involved. The means employed to accomplishthis, result comprises the `elements at lthe right-hand end of Figure 7.If the selector switch 44 is connected to the same divider as" theswitch 6a, and if the switch 45 is closed, the pulses of light whichwill be emitted from the crater lamp 48 will not occur as often as thepulses produced by the lamp 8c.

For example, if the frequency divider 43 is a systemV which will divideby 5, then the extended portions 73a of the graduations in Figure 13will appear on every 5th one of the graduations produced by the exposureof the plate 42 by the lamp 8c. It will be obvious that an additionalmeans similar to that comprising the elements between the selectorswitch 44 and the mirror 49, but con-` nected to a Vdifferent divider,can be employed to produce additional prolongations 74a of thegraduations already obtained by the system just described.

As'another procedure, I may index a series of graduationsrby exposingfirst the normal areas 72; shifting the aperture 39!) of Figure 9 to aposition corresponding to that of the extended portion 73a of agraduation 72a; third, selecting a different dividing ratio; and fourth,exposing a new series of areas to obtain prolongations 73a ofgraduations 72a. described system for obtaining exposures which arecoradial will have to be employed in order that the ex'- V tendedportions 73a are properly aligned with the graduaspond to a pulsefrequency with which the cycles 72C,

72d, etc. of a desired wave form are synchronized.

y Harmonic synthesizer In the synthesizer, I use a method of thepulse-sampling type. Although systems employing this general method aredescribed in an article entitled Method for changing frequency of acomplex wave in the Proceedings of the National Electronics Conference,Chicago, Illinois, 1946, l prefe r to use a diode system together withmeans for controlling of amplitude and phase of the harmonics withrespect to vthe .fundamental frequency, which I believeis..

novel., inorderV that I can meet objects of my invention pertaining tothe provision during one revolution of a disc, or the like, ofV anintegral number of cycles of various wave forms. Y

.The harmonic synthesizer, represented by the block 13.0f Figure 2, isshownin greater detail in the area surrounded by the dot-dash line 13aof Figure l5.. The

synchronizing circuit, enclosed in the area 14a will be described indetail below in another section.

In Figure l5, a crystal oscilaltor 75, generating a relatively highfrequency signal'of the order of lOll kilocycles per second, isconnected toa pulse generator or a blocking oscillator 76 for control ofthe oscillation frequency thereof at a frequency equal to or asubmultiple of the crystal oscillator. The harmonically-richoscillations from the blockingv oscillator 76 are fed to a series ofparallel filters, twoof which are illustrated generally atr77 and 77a. i

The blocking oscillator output enters each filter through a seriesresistor, such as 85, a, connected in series to ground with apotentiometer 82, 82a, etc. The complex potential, is fed therefrom toone tuned circuit 78, 78a,

in each filter through a resistor 79, 79a and to another The Y tunedcircuit 80, Silla through a capacitor 81, 81a. two tuned circuits 7 8and 80 of iilter 77 are tuned to the fundamental frequency of theblocking oscillator output; 'tlie Vtuned circuits 78a and 80a of theiilter 77a are tuned to the second harmonic of the blocking oscillatoroutpii'tgand so on. The axes of theiiidubtive 'elements of 'the tunedcircuits 78 and 80 are' 90 degrees apart and It will be obvious that theabove-V their respective voltages are 90 degrees apart, so that whenapick-up element 87 is rotated, the phase of the induced E. M. F. isshifted uth respect to the blocking oscillator output, hence withrespect to other harmonics derived from other filters, such as 77a. Theamplitude of the E. M. F. in pick-up element 87 can be controlled by thepotentiometer 82. The purpose of the resistor 85 is to provide means foradjusting the maximum amplitude of the E. M. F. derived from the pick-upelement 87. The capacitors 81, 81a may be varied to match the resistors79, 79a in equalizing the E. M. F. induced in pick-up element 87 in itsvarious positions of rotation.

The inductive elements 88, 88a of each filter, in conjunction with theinductive elements 87, 87a provide link coupling between the filters 77and 77a respectively and tuned pick-up circuits 89, 89a. These tunedcircuits are connected in series as shown so that the resultant outputmay be fed to a broad-band amplifier 90, capable of passing the highestdesired harmonic of the blocking oscillator output. Means is thusprovided for combining and amplifying harmonically-related E. M. F.s indifferent amplitudes and phase relationships for the production at thefrequency of the blocking oscillator 76 of complex oscillations of anydesired wave form.

Now comes the problem of changing the frequency of the wave thusproduced to a desired frequency without changing substantially itsharmonic content. I accomplish the change by the general methodhereinbefc-re referred to.

A variable oscillator 95 determines the frequency of pulses generated bya pulse generator 96. This frequency is preferably variable throughout arange such that the difference between it and that of the blockingoscillator 76, may be varied from zero to whatever maximum fundamentalfrequency it is desired to produce from the synthesizer. Negative pulsesfrom the generator 96 are mixed with the complex wave output from thebroadband amplifier 90, as shown, and are fed to the cathode of a vacuumdiode 97, which cathode is maintained at a positive potential withrespect to ground by suitable means connected at 9S. Since the plate ofthe diode 97 is at ground potential, except when current is owingtherethrough, the diode will conduct whenever the cathode is drivennegative. Consequently, if the relationship between the complex wave,the positive bias and the pulse are such that the pulse drives thecathode negative during each pulse occurrence, the pulses of currentthrough the diode 97 will be samples of the complex wave, taken at aslightly different point in each cycle of the complex wave. Providing alow-pass filter 99 greatly attenuates the pulse frequency, its harmonicsand frequencies as low as one-half the pulse frequency and providing theone-half the pulse frequency, then the output of the filter will containonly the desired difference frequency and its harmonics and will be ofsubstantially the same wave form as the original complex wave.

The wave thus produced may be amplified by an amplifier 100 and employedto modulate a crater lamp 8d or the like for the purposes discussedherein.

It will be obvious however, that the synthesizer itself has utilityother than that discussed herein. For example, the output may beconnected to an oscilloscope and to an electroacoustic translatingsystem for demonstrating the sound of various complex waves.

Synthesizer synchronizing system In order to produce an integral numberof cycles of a desired wave form during one revolution of a turntable orthe like, so that a wave form or voice disc can be produced for use in aphotoelectric musical instrument, it is necessary that the synthesizerused to produce a desired wave form be synchronized with the motordriving the turntable.

Since I have already described a system by means of which I can produceone revolution of a turntable during the occurrence of an integralnumber of periodic electric pulses, it will be obvious that if Isynchronize the desired wave with the pulses, integral number of cyclesof a desired wave form may be produced during one revolution. Theessential elements of a system for accomplishing this result are shownin the area enclosed by the line 14a in Figure 15. What I do is tocompare the frequency of pulses from a pulse generator such as 5a inFigure 7 with the difference between the fundamental frequency of theblocking oscillator 76 and the frequency of the variable oscillator 95.If they are not the same, I vary the frequency of the variableoscillator until they are the same, employing the following means:Referring to Figure 15, an L-C circuit 110, tuned to the fundamentalfrequency of the blocking oscillator 76, is connected as shown through aresistor 111 to the blocking oscillator. The voltage across circuit isfed through an amplifier 112 to the cathode of a vacuum diode 113 asshown. To the plate of the diode 113, through a small coupling capacitor114 is fed a square wave from the variable oscillator 95, The resultantwave at the plate of the diode 113 is fed through a lowpass filter 115similar to filter 99. The output of the filter 115 is a sine wave at thedifference frequency between the fundamental of the blocking oscillator76 and the variable oscillator 95. After amplification by an amplifier116, the sine wave is mixed at the point 117 with the pulses with whichthe synthesizer is to be synchronized, and fed through a diode circuit118 used to bias a reactance tube 119. If there is a difference betweenthe pulse frequency and the difference frequency mixed at 117, the phaseshift causes the D. C. level biasing the reactance tube to change and tovary the impedance offered by the tube across the tuned circuit of thevariable oscillator 95, until the difference is zero.

Thus means have been provided for producing an integral number of cyclesof a desired wave form during arevolution of a turntable or the like. Itwill be obvious then that I may employ this system in the production ofvoice discs of the type described in my copending application entitledMethod and Means for Producing Tones and Voices Photoelectrically,Serial No. 117,239, filed September 22, 1949, now U. S. Patent No.2,576,759 granted November 27, 1951. Such voice discs have a pluralityof substantially radial series of wave-form patterns corresponding tothe respective tone colors of a photoelectric musical instrument. Eachseries contains one or more cycles of a wave form for each concentricseries of scanning slots in a rotating pitch disc. It will be obviousalso that in order to provide wave-form patterns whose individual cyclelengths are exactly equal to the distance between scanning slots, itwill be necessary to produce during one revolution of the disc uponwhich the wave forms are exposed, a number of cycles equal to the numberof scanning slots in the corresponding series on the cooperating pitchdisc. In the production of a wave form disc, I may mask that portion ofthe disc which is not to be exposed, leaving a space equal to a desirednumber of cycles for exposure by a light source whose intensity isvaried in accordance with the desired wave form, or I may expose acomplete circle of wave form patterns, using them as a master from whichI make my wave form disc by exposure through the master of a desirednumber of cycles.

It will be obvious to those skilled in the art that the successfulproduction of equally and accurately spaced narrow graduations producedby the methods described above, will depend upon a motor which revolvesat an extremely constant speed.

In Figure 16, is illustrated such a motor. The rotor element 124 of thesynchronous motor having an axle 125 is journaled for rotation by ballbearings 126 and 127 in a stator clement 128 mounted to a rigid support129 by screws such as 130. The internal surface of the rotor 124 and theexternal surface of the stator y128 have equal numbers of rectangularshaped teeth such as indicated at 131 and 132 respectively. The windings133 and 134,'to which the pulsating D. C. is supplied, are located inthe stator as shown. The turntable 12b is mounted upon the top face ofthe rotor element 124 by means of screws such as 135. In order to reducehunting in the turntable to a minimum, the turntable comprises a plateequipped with bosses 136 surrounded by a tubular member 137, preferablya standard bicycle tire which is filled with a viscous liquid such asglycerine as at 138. As is known in the art, the liquid-filledmember.137 acts as a viscous damping means for reducing to a minimumperiodic departures from synchronous speed caused by the rotor 124hunting about its synchronous position with respect to the stator 128.

If there are any minor inaccuracies in the construction of the motor,which tend to produce periodic departures from synchronous speed, onceduring each revolution, a means must be provided to compensate for suchdepartures; otherwise cumulative departures of graduations produced bymeans of the motor and a fiashing light would result. I have shown sucha means in Figure ,16,

tuned respectively to the fundamental pulse frequency and its harmonics,said filters including variable irnpedance means and variable couplingmeans for controlling respectively the amplitude and phase relationshipbetween said harmonics and the fundamental, a plurality of tunedcircuits connected in series and coupled respectively to said filters, avariable oscillator, a second pulse generator' in connection therewithand generating pulses at a frequency determined by said variableoscillator, a mixer circuit connected between said tuned circuits andsaid second pulse generator for mixing the resultant voltage of saidcircuits with the pulses from said second generator, and a low-passfilter coupled to said mixer circuit and operative to attenuate allfrequencies above substantially one-half the frequency of the pulsesfrom said first pulse generator.

2. Means for synchronizing a synthesizer whose output frequency is equalto the difference between the frequency Vof its pulse generator and thefrequency of its variable frequency oscillator with the frequency ofperiodic pulses from an outside source, comprising in coms bination atuned circuit in connection with said pulse wherein a spring member 139Vis supported under tension between a fixed pin 14D and a peg 141eccentrically mounted to the axle 125 by an element such as the disc142. It will be obvious that the peg 141 will be located at such a pointand that the spring of 139 will be of such strength as to counteract theacceleration and deceleration caused by the mechanical inaccuraciesreferred to above.

Should the mechanical inaccuracies result in departures from synchronousspeed which are not of a simple nature, a cam-actuated system may beemployed to compensate for the complex variations. j

I have, as mentioned above, produced a circular trace on the tube T8 ofFigure 7 for convenience in counting markers or pips for ascertainingdividing ratios between the low and high frequency oscillations. It willbe obvious, however, that a sweep other than a circular one may beemployed in such a system as described herein, once an approximate ratiohas been established. For example, a horizontal trace could be producedin the usual manner, superimposing thereon vertical pips produced by themeans shown in Figure 7.

In the description of my invention, 1 have referred to certain componentcircuits by names current in the art and readily recognized by theskilled worker. For the purpose of a complete disclosure, I may statethat I may employ a phase shift network such as that illustrated inFigure 53, chapter l5 of Theory and Application of Electron Tubes byReich, McGraw-Hill, second edition, 1944; and I may employ a liip-iiopmultiple vibrator, such as that illustrated in Figure 7, chapter l() ofthe same volume and described in connection therewith; that the variouspulse generators referred to hereinabove including the so-called widepulse generator may be circuits, such as shown in Figure 16, chapter l()of the same volume; that a blocking oscillator as employed by me mayhaveV the construction illustrated in Figure 19, chapter 13 of RadarSystem Engineering, edited by Ridenour, McGraw-Hill, first edition,1947; and that the amplifier and mixer 26a. employed by me embodies acircuit such as two conventional triode amplifiers, the plates of whichare connected in parallel to an output circuit, the oscillations to bemixed being fed to the'respective grids of the tri-ode amplifiers.

Modifications may be made in my invention without departing from thespirit of it. Having described my invention in certain exemplaryembodiments, what I claim as new and desire to secure by Letters Patentis:

l. In a. harmonic synthesizer, the combination of a source of fixedfrequency oscillations, a first pulse generator in connection therewith,the frequency of the pulses Series of filters coupled to said firstpulse generator and generator and tuned to the fundamental frequency ofthe pulses generated thereby, means coupled to said tuned circuit foramplifying the potential across said tuned circuit, a diode circuit inconnection with said variable frequency oscillator an-d said amplifyingmeans for mixing the respective o-utputs thereof, a filter in connectionwith said diode circuit for passing the difference frequency betweensaid generator and saidoscillator, amplifying means in connection withsaid filter for amplifying said difference frequency, a second diodecircuit coupled to said last-mentioned amplifying means and said outsidesource yfor mixing the output of said last-mentioned amplifying meansand said pulses from said outside source, a reactance tube circuit inconnection with said diode 'circuit and biased thereby by a potentialwhich is directed proportional to the difference in phase between thefrequency of said outside pulses and said difference frequency, saidreactance tube being in connection with said variable oscillator andoperative to modify the frequency generated thereby to bring thedierence frequency into equality with said outside pulse frequency.

3. A system for producing a complex wave of a desired form whosefundamental frequency component is at the same frequency as that ofcontrol pulses from an outside source, comprising a harmonic synthesizerand a synchronizing circuit connected between said outside source andsaid synthesizer, said synthesizer comprising the lcombination of asource of fixed frequency oscillations, a first pulse generator inconnection therewith, the frequency of the pulses generated therebybeing determined by said last-mentioned source, a series of filterscoupled to said first pulse generator and tuned respectively to thefundamental pulse frequency and its harmonics, said filters includingvariable impedance means and variable coupling means for controllingrespectively the amplitude and phase relationship between said harmonicsand the fundamental, a plurality of tuned circuits connected in seriesand coupled respectively to said lters, a variable oscillator, a secondpulse generator in connection therewith and generating pulses at afrequency determined by said variable oscillator, a mixer circuitconnected between said tuned circuits and said second pulse generatorfor mixing the resultant voltage of said -circuits with the pulses fromsaid second generator, and a low-pass filter coupled to said mixercircuit and operative to attenuate all frequencies above substantiallyonehalf the frequency of the pulses from said first pulse generator,said synchronizing circuit comprising the combination of a tuned circuitin connection with saidfirst pulse generator and tuned lto .thefrequency of the pulses generated thereby, means for amplifying thepotennal across said tuned circuit, a diode circuit in connection withsaid kvariable oscillator and said amplifying means for mixing therespective outputs thereof, a filter in connection with said diodecircuit for passing the diterence frequency between said pulse generatorand said oscillator, amplifying means in connection with said filter foramplifying said dierence frequency, a second diode crcuit for mixing theoutput of said last-mentioned amplifying means and said pulses from saidoutside source, a reactance tube circuit in connection with said diodecir cuit and biased thereby by a potential which is directlyproportional to the diierence in phase between the frequency of saidoutside pulses and said difference frequency, said reactance tube beingin connection With said variable oscillator and operative to modify thefrequency generated thereby to bring the difference frequency intoequality with said voutside pulse frequency.

References Cited in the le of this patent UNITED STATES PATENTS2,075,802 Davis Apr. 6, 1937 2,121,359 Luck et al. June 21, 19382,164,809 Fisher July 4, 1939 18 Norton Mar. 11, Mathes et al. Mar. 3,Schlesinger lune 6, Hassler Feb. 18, Jones Apr. 13, Ranger Nov. 2,Hallmark Jan. 18, Bliss Dec. 6, Young Dec. 6, Goldberg Feb. 7, Wood Mar.28, Grosdoff Sept. 12, Sziklai Nov. 28, Hugenholtz Feb. 6, Wickham Feb.6, Ranger Feb. 27, McFarlane June 12, Hammond Aug. 7, Hester Apr. 22,Gray June 24,

Garman et al. July 22,

